Conventionally, a water absorbing material made of a hydrophilic fiber such as pulp, and a water absorbing agent has been in widespread use in sanitary materials such as a disposable diaper, a sanitary napkin, and an incontinence pad, for the purpose of absorbing body fluids.
For enhancement in convenience, it has been demanded to make these sanitary materials thinner in recent years. This results in that, in a water absorbing material, a ratio of the hydrophilic fiber having low bulk specific gravity is decreased, and a ratio of the water absorbing agent having a high water absorbing rate and high bulk specific gravity is increased. Water absorbing agent usage in the water absorbing material is thus increased so as to make the sanitary material thinner without a reduction in its properties such as a water absorption amount.
Such a sanitary material having a small ratio of the hydrophilic fiber and a large ratio of the water absorbing agent is preferable for merely keeping liquid. However, when actually used in a disposable diaper, for example, such a sanitary material has a problem in distribution and diffusion of the liquid. For example, a large amount of the water absorbing agent turns into a soft gel state when absorbing water, and causes a gel blocking phenomenon in which water is prevented from being absorbed deeper into the water absorbing agent. This dramatically decreases a diffusing property of the liquid in the sanitary material. The ratio of the hydrophilic fiber to the water absorbing agent is inevitably limited so as to both avoid such a problem and maintain absorption characteristics of the water absorbing body. It follows that the sanitary material cannot be thinner than a certain limit.
In order to both suppress the gel blocking and realize a sufficient absorption amount, it is necessary to obtain a water absorbing agent that is excellent in a balance between an absorption capacity represented by a centrifugal retention capacity (CRC), for example, and liquid permeability represented by a saline flow conductivity (SFC), for example. However, they have such a relationship that an increase in one results in a decrease in the other. This makes it difficult to improve the relationship (balance) between them to a successful level. As means for attaining such an object, the following techniques have been known, for example.
Patent Document 1 discloses a water absorbent resin that has been processed with a tri- or more-valent cation. Patent Document 2 discloses a technique with which an electrostatic or stereoscopic spacer is used with a water absorbent resin. Patent Document 3 discloses a particulate water absorbing agent containing: water absorbent resin particles produced by (i) cross-linking a monomer containing acrylic acid and/or salt thereof, and (ii) further cross-linking a surface of each of particles thus obtained, which particles have been pulverized into irregular shapes; and an agent for enhancing liquid permeability. An object of Patent Document 3 is to provide a water absorbing agent having properties of both capillary suction pressure and liquid permeability.
Meanwhile, other than Patent Documents 1 through 3 whose object is to improve the balance between the absorption capacity and the liquid permeability, there has been proposed to mix various additives for the purpose of an improvement in various properties of a water absorbent resin.
For example, Patent Document 4 discloses a method of producing a water absorbing agent, including: the steps of: processing surface of water absorbent resin particles with a cross-linking agent, the water absorbent resin particles having a carboxyl group and the cross-linking agent having two or more functional groups that can form a covalent bond by reacting with the carboxyl group, so as to crosslink a part of the carboxyl group; and then mixing the particles with a cationic polymer compound having a weight-average molecular weight of 2000 or more, which cationic polymer compound can form an ion bond by reacting with the carboxyl group. Thereby, Patent Document 4 easily obtains a water absorbing agent that hardly moves or is omitted in complex with a cellulose fiber, which water absorbing agent has such a feature so as to be expected to have a synergetic effect in absorption performance.
Further, Patent Document 5 discloses a water absorbing agent composition containing water-insoluble inorganic powder fine particles and/or a polyamine compound having a weight-average molecular weight of 5000 or more, wherein: under a load of 20 g/cm2 (1.96 kPa), 0.9 weight % sodium chloride solution has a diffusing absorption capacity of 25 g/g or more after a lapse of 60 minutes from the beginning of absorption.
Furthermore, Patent Document 6 discloses a water absorbing agent composition containing: water absorbent resin particles having at least an anionic dissociable group; and water-swelling resin particles having a cationic group. Thereby, Patent Document 6 provides a water absorbing agent composition that is more excellent in an absorption speed, a permeability rate of a gel layer, a permeability rate of a gel layer under pressure, and an absorption capacity against pressure respectively, than each of the two kinds of the resin particles solely used.
Patent Document 7 discloses water absorbent resin particles, wherein: at least a part of a surface of the water absorbent resin particles is coated with a coating film that is made from an adduct of a polyvalent amine compound and a compound having an olefinic double bond. Thereby, Patent Document 7 provides: a water absorbing agent that is advantageous in an absorption capacity under pressure or no pressure, provides high safety for a human skin and the like, and is suitable for use in a sanitary material; and a method of producing the water absorbing agent.
Further, Patent Document 8 discloses a mixture of water absorbent resin particles and a cationic polymer compound in which a specific region is crosslinked. Thereby, Patent Document 8 realizes that, even long after a swelling, it is possible to (i) keep a state where gels clump together and retain a shape, and (ii) have high liquid permeability and a high absorption capacity against pressure.
Patent Document 9 discloses a water absorbing agent made from a nitrogen-containing polymer having a nitrogen atom that can realize a protonation of 5 to 17 mole/kg against a total mass of particles of a water absorbing agent polymer, and the nitrogen-containing polymer. Thereby, Patent Document 9 provides a water absorbing agent having: an improved property of high absorption performance; improved liquid permeability; and high water resistance.
Patent Document 10 discloses a water absorbent resin produced by compounding an acid crosslinked polymer and a basic crosslinked polymer. In Patent Document 10, a crosslinked polyethylenimine, and a crosslinked polyallylamine are employed as the basic crosslinked polymer.